Marching and clapping rhythmically to show they come in peace and are not brandishing weapons, the visitors to Castell Henllys Iron Age Fort, Pembrokeshire, ascend its defensive earthworks. Suddenly, eardrum-shattering yells break out above them and within seconds they are in the middle of an ambush perpetrated by the chieftain Culhwch and his fierce, woad-faced followers.

But the chieftain is soon mollified when eight-year-old Christopher restores his long-lost sword to him - a task entrusted by the gods to these children from Clase Junior School, Morriston in Swansea. “Welcome to my village, ” he says, and, leading them to the largest roundhouse, asks what adventures they have had along the way.

Their adventures had begun 2,000 years into the future at the gateway to the Trail of the Twrch where they were greeted by Culhwch’s sister, Rhaonwen Owen (education officer) attired in the woollen dress and shawl of an Iron Age lady. Leading them along, she related Culhwch’s history as told in the ancient Mabinogion tales - a direct link into Celtic culture and superstition.

The children heard that when Culhwch was young, handsome and brave he wished to marry the beautiful Olwen. But Olwen’s father the giant Ysbaddaden, knowing he was destined to die when she married, set Culhwch several “impossible” tasks. Unfortunately for him, Culhwch succeeded and chopped off Ysbaddaden’s head, which “fell in this exact spot”. Looking around in disbelief, the children saw a face materialising in a gnarled tree trunk with an enormous misshapen boxer’s nose and bushy moustache.

Culhwch’s final task had been to take a golden comb and scissors from a gigantic boar. King Arthur’s knights helped him chase the boar over the sea from Ireland to this Welsh wood, where there was a bloody battle. At last Culhwch won and threw his sword in the river as a thanksgiving to the gods. Two thousand years have passed and he is now old and cantankerous, but he still hopes the sword will be returned to him one day.

Before they could accompany Rhaonwen Owen to her village the children had to travel back 2,000 years by walking and chanting through a Celtic maze called the “Time Machine”. (Half of them - future ambushers - went on ahead and the others followed later with Rhaonwen.) No sooner had they passed over the bridge into the Iron Age than several cries went up: “There’s a sword in the river!” This they bore to the Sacred Spring, which, Rhaonwen explained, “is both the doorway to other worlds and where the gods visit our world - the Celts had hundreds of gods in trees, rivers, lakes, mountains and wild animals”. To attract them, scraps of bright material adorn the trees, particularly on the special feast days: Sammhein, Lughnasad, Beltaine and Imboc. Today, the deer god Cerminos was awaiting them, bearing Culhwch’s scabbard, into which the sword fitted easily.

Close by the hill fort, kept safe from raiders, the children found animals of similar breeds to those existing in the Iron Age: a male pig with incipient tusks, a sow, goats and pretty Solway sheep, with dark grey wool which can be picked off. The children fed them, laughing as the greedy sow snorted loudly and fought the male for his share. Here, swearing them to secrecy, Rhaonwen revealed her special names for the pigs . . . Culhwch and Olwen. “Sometimes when I hear the real Culhwch (Tim Bowley, actorstoryteller) roar,” she confided, “I know they’ve let the cat out the bag. I embroider what I tell them and Tim says he never knows what he’s going to hear next.”

As they proceed to the fort, Rhaonwen explains that its occupants chose this location because it could be easily defended and enemies could be seen approaching. And she talks about the social order - chieftains and tribes - and cultural traditions such as music, poetry, art and mythology. Finally, the children practised their yell of greeting (until it’s deafening enough to alert Culhwch and the villagers to their arrival!).

The ambush over, the children explore one of the roundhouses. An authentic charcoal fire has been lit within. The roundhouses have been carefully reconstructed, so Culhwch is able to demonstrate exactly how they were built and divided into sleeping, living and kitchen areas.

The children then divide into groups for activities such as pottery, face painting, spinning, weaving, spear-throwing competitions, and building wattle hurdles by weaving wood between stakes and daubing on a cocktail of clay, straw, horsehair and cow dung.

Class teacher Joanne Larcombe says the day will inspire half a year’s study on the Celts, which is on the Welsh KS2 curriculum, and support teacher Marlene Martin says: “the children have been transported and really involved.”

According to site director Phil Bennett, Pembrokeshire Coast National Park Authority purchased Castell Henllys “to prevent it getting into the hands of the developers or reverting to farmland”. New facilities include an award-winning education centre, Llys Annwn (court of the Celtic Otherworld). “We can now cater for an age range of six to 60,” he says. “Many teachers have said a visit here can dictate a term’s work.”

Castell Henllys Iron Age Hill Fort, Meline, Crymych SA41 3UT. Tel: 01239 891319 * Two packs, Pathways to the Past: the Celts at Castell Henllys and Adventures in Design and Technology Structures, have been produced by the National Park in conjunction with Dyfed LEA. They contain videos and worksheets. Pounds 23.95 each.